1. Field of the Invention
The present invention relates to a process for producing a thin film of strontium titanate (hereinafter referred to as STO), barium titanate (hereinafter referred to as BTO) or barium strontium titanate (hereinafter abbreviated as BST) by chemical vapor deposition (CVD) using a specific titanium compound as a precursor and to a CVD precursor comprising the specific titanium compound.
2. Description of Related Art
Thin films of a titanium-alkaline earth metal composite oxide typified by STO, BTO or BST are expected for application to semiconductors and electronic components because of their unique electrical characteristics, such as high dielectric constants. In particular, their application to semiconductor memories as a capacitor has been under study.
Processes for making a thin film of STO, BTO or BST include CVD, flame hydrolysis deposition, sputtering, ion plating, and metal-organic decomposition (MOD) such as dipping-pyrolysis process and a sol-gel process. Among these thin film forming technologies, CVD is the most suitable process because of ease of composition control, excellent step coverage, suitability to large volume production, and capabilities of hybrid integration, as is reported, e.g., in JP-A-6-158328.
Ti source precursors fit for CVD include titanium tetraalkoxides, e.g., titanium tetraisopropoxide. However, when they are combined with St- and/or Ba-supplying precursors for CVD, it is difficult to establish strategy for stable and industrial production of STO, BTO or BST thin films having ease of composition control and excellent step coverage on account of mismatch between the Ti source precursors and the St and/or Ba source precursors in terms of vaporization characteristics or decomposition behavior.
Ti source precursors for CVD also include titanium complexes having a xcex2-diketone as a ligand. For example, JP-A-5-271253, JP-A-9-40683, JP-A-10-72475, JP-A-10-114781, JP-A-11-199591 and JP-A-11-255784 disclose titanium compounds having an alkyl-terminated xcex2-diketone represented by 2,2,6,6-tetramethylheptane-3,5-dione. These materials are also unsatisfactory for the same reasons as described above. They involve an additional problem that the titanium content in a resulting thin film falls short of what is designed.
An object of the present invention is to provide a process of forming an STO, BTO or BST thin film by CVD which can be performed stably enough for industrialization and to provide a precursor for CVD.
As a result of extensive investigation, the present inventors have found that the above object is accomplished by a titanium compound having a specific molecular structure and completed the present invention.
The present invention provides a process of producing an STO, BTO or BST thin film by CVD, which comprises using a titanium compound represented by formula (I): 
wherein R1 represents a hydrogen atom or a methyl group; and R2 and R3 each represent a methyl group or an ethyl group, or R2 and R3 are connected together to form a methylene group, a methylmethylene group or a dimethylmethylene group.
The present invention further provides a precursor for CVD comprising a titanium compound represented by formula (III): 
The present invention furthermore provides a precursor for CVD comprising a titanium compound represented by formula (IV): 